1. Field of the Invention
The present invention relates to projection optical systems and projection apparatuses and, more particularly, to a projection optical system and a projection apparatus suited to be used with a single-panel color liquid crystal display device, in which three light beams of R (red), G (green) and B (blue) colors illuminate an array of picture elements on a liquid crystal display panel at different angles of incidence, and all of the light beams exiting from these picture elements are combined in passing through different pupils in a position of a common projection lens and are projected onto a screen.
2. Description of Related Art
The display apparatus (projection apparatus) well known in the past employs a plurality of liquid crystal display panels or like display elements and projects the images of different original colors in the respective panels onto a screen where a picture is produced in the whole gamut of colors. Recently, as the resolution of the image to be projected has been becoming ever higher, there is a strong demand for reducing the bulk and size of the whole apparatus. For the projection lens, too, it is desired to have its size reduced and its imaging performance improved accordingly.
Usually, in order to make the brightness and definition of the projected picture to be high at once, the white light from a light source is separated out to three light beams of original colors R, G and B, and three display elements are employed for producing the corresponding images to the color separation. These three images of different colors formed on the three display elements are combined by one projection lens and are projected onto the screen. Such a projection apparatus is of the so-called three-panel type.
In this case, for the light source, there are many incandescent lamps such as halogen, xenon and metal halide ones. The more luminous the lamp to be used, the brighter the projected picture on the screen becomes.
The three-panel projection apparatus can advantageously utilize the illuminating light from a common white light source and, therefore, obtain a picture nearly three times brighter than that obtained from a single panel of liquid crystal display despite the use of three original colors.
Further, concerning even with the increase of the definition, if the three liquid crystal display elements of the same picture element number in the light beams of colors, R, G and B are mixed by the law of addition strictly on the screen, one set of picture elements of different colors can reproduce a perfect white. Actually, however, the projection lens that combines the three color light beams in projection has lateral chromatic aberration, causing the three picture elements of different colors in each set to be taken out of coincidence, in short, a color offset to occur. How to solve this is a problem as has been put on the projection lens.
In another aspect, for the three-panel type, because the structure of construction becomes far more complicated than that of the single-panel one, there are also disadvantages of hardly manufacturing it economically and increasing the size.
To improve the above disadvantages of the three-panel type, it has been known in the art to provide a projector of the single-panel type in a simple form with the brightness increased, as disclosed in, for example, Japanese Patent No. 2622185, the constituent Em -parts of which are shown in FIG. 7.
In FIG. 7, a white light source 41 is backed by an elliptic reflector 42. A condenser lens 43 is positioned with its object point at the focus of the elliptic reflector 42 to produce parallel light beams. In front of the condenser lens 43 there are arranged three dichroic mirrors 44R, 44G and 44B having optical properties of selectively reflecting light beams of wavelengths in red (R), green (G) and blue (B) regions respectively, leaving the other light beams to pass therethrough. The red and blue dichroic mirrors 44R and 44B each make an equal angle of xcex8 degrees but in opposite direction with respect to the green dichroic mirror 44G.
Therefore, the reflected rays of light converge to an angle of 2xcex8, arriving at a liquid crystal display element 46.
A micro lens array 45 is positioned just in front of the liquid crystal display element 46 and mounted in unison therewith. Micro lenses constituting the micro lens array 45 are so arranged that the color light beams R, G and B separated out by the dichroic mirrors 44R, 44G and 44B each illuminate the ones of the pixels of an image which are driven for the same color at a time by the color signal. This principle will be described more fully later. The light beams that have passed through the respective pixels pass through different pupils 48R, 48G and 48B in position of a projection lens 47, forming a picture on a screen 49.
FIG. 8 is a schematic sectional view of the conventional example of the liquid crystal display element 46 and the micro lens array 45 shown in FIG. 7. The liquid crystal display element 46 is composed of two glass plates 52 and 53 with a liquid crystal layer 50 sealed therebetween. Responsive to control signals for the color light beams R, G and B, electrodes 51R, 51G and 51B in the liquid crystal layer 50 turn on and off the pixels. All the pixels are arranged in a mosaic or stripe fashion.
The color light beams emerge from the pixels at different angles like the different angles of incidence, entering the projection lens 47. On the pupils 48R, 48G and 48B of the projection lens 47. Therefore, the useful areas of the light beams slightly differ with different colors as shown in FIG. 7.
As a result, the projection lens 4 is required that its value of F-number be smaller than that for the three-panel type, that is, to be brighter.
In the three-panel projection apparatus, if some lateral chromatic aberration remains in the projection lens for projecting light beams from a color liquid crystal display element onto a screen, the perfect agreement of the R, G and B pixels on the liquid crystal element does not assure avoidance of producing a color offset on the screen, deteriorating the picture quality. It is now assumed that, in the projection lens, there are lateral chromatic aberrations for the three colors as shown in FIG. 9, where the lateral chromatic aberration is in the abscissa and the image height on the screen is in the ordinate.
In FIG. 9, for the marginal zone of the liquid crystal display element, the lateral chromatic aberration is plus in the B light and minus in the R light as compared with that in the G light. It is to be noted that the liquid crystal display surface is taken as the object surface, and the screen is taken as the image plane. That is, on the screen, any picture element for the R light that should overlap the corresponding picture element for the G light as a rule shifts in a direction to the center of the screen, while any picture element for the B light shifts in the reverse direction. It is to be noted that, in consideration of the projection lens only at its influence, the left and right halves of the screen are in point symmetry with respect to the optical axis of the projection lens. So, there is no asymmetry between the left and right marginal zones. Usually, the green light is most intense and looks prominent, and the blue light is lower in relative luminous efficiency than even the red light. This situation is depicted by exaggeration in FIG. 10.
On the other hand, for the single-panel type, it is supposed that the projection lens having the same lateral chromatic aberrations as those in FIG. 9 is in use. The pixels in the panel are assumed to have a stripe-like arrangement such that the pixel for the G light is flanked by the left and right pixels for the R light and B light. This order may be exchanged without causing any affection to the essence.
In FIG. 11, similar to FIG. 10, the shift of the picture elements on the screen is shown. With the green picture element 81G as a reference in the left marginal zone, even if the projection lens has no lateral chromatic aberration, the spread of the pixels in only one plane causes displacement of the red and blue picture elements to positions 81R and 81B, respectively, with the center of each of the elements marked xe2x80x9c+xe2x80x9d. If, in addition, the lateral chromatic aberration of the projection lens is further taken into account, the picture elements 81R and 81B are shifted to positions 81Rxe2x80x2 and 81Bxe2x80x2, respectively. Thus, a color offset occurs.
Conversely, looking at the right marginal zone, the red and blue picture elements are displaced from positions 82R and 82B which are ideal in respect to the position 82G of the green picture element as the reference, to positions 82Rxe2x80x2 and 82Bxe2x80x2, respectively. Thus, the color offset in the right marginal zone is asymmetric to that in the left marginal zone. This also constitutes a drawback intrinsic solely to the single-panel type color liquid crystal display device.
Then, similarly, the lateral chromatic aberration of the projection lens is considered. Since the lateral chromatic aberration of the projection lens is in point symmetry with respect to the optical axis of the lens, the picture elements shift in the same direction as that in the left marginal zone. As a result, in the right marginal zone, the G, R and B light beams shift in the good direction, overlapping each other, and any color offset comes to hardly occur. In this case, however, the symmetry between the left and right marginal zones collapses extremely, and, therefore, this drawback becomes objectionably conspicuous.
The present invention, in application to the projection optical system for the single-panel type display element, has set forth proper rules for the lateral chromatic aberration of the projection lens constituting part of the optical system. It is, therefore, an object of the invention to provide a projection lens and a projection apparatus which diminish the asymmetry of color offset between the left and right marginal zones of the screen, making it possible to view pictures of good quality.
To attain the above object, in accordance with an aspect of the invention, there is provided a projection optical system for projecting onto a screen an image formed on a display device having a plurality of pixel sets each of which is composed of at least three pixels arranged in correspondence to at least three color light beams of respective different wavelengths, the projection optical system comprising a projection lens having a plurality of lens elements, wherein the projection lens is so formed that lateral chromatic aberration for a color light beam of a wavelength longer than that of a predetermined color light beam has the same sign as that of lateral chromatic aberration for a color light beam of a wavelength shorter than that of the predetermined color light beam.
Further, in accordance with another aspect of the invention, there is provided a projection apparatus, which comprises a display device having a plurality of pixel sets each of which is composed of at least three pixels arranged in correspondence to at least three color light beams of respective different wavelengths, and a projection lens for projecting onto a screen an image formed on the display device, wherein the projection lens is so formed that lateral chromatic aberration for a color light beam of a wavelength longer than that of a predetermined color light beam has the same sign as that of lateral chromatic aberration for a color light beam of a wavelength shorter than that of the predetermined color light beam.
The above and further objects and features of the invention will become apparent from the following detailed description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.